A chalcopyrite type solar cell is a solar cell which includes, as a light absorption layer, a chalcopyrite compound represented by Cu(InGa)Se (hereinafter also referred to as “CIGS”). Special attention has been focused on this type of solar cell, since chalcopyrite type solar cells possess various advantages. For example, the energy conversion ratio of such solar cells is high, the optical deterioration due to aging is rarely caused, resistance to radiation is excellent, and the solar cell exhibits a wide light absorption wavelength range as well as a large light absorption coefficient. Thus, various investigations have been conducted in order to realize mass production of chalcopyrite type solar cells.
As shown in FIG. 5, a chalcopyrite type solar cell 10 of this type is provided by disposing a stack 14 on a glass substrate 12. The basic structure of the stack 14 includes a first electrode 16 composed of Mo, a light absorption layer 18 composed of CIGS, and a transparent second electrode 20 composed of ZnO/Al. However, in general, a buffer layer 22 and a high resistance layer (semi-insulative layer) 24 are also provided, which intervene between the light absorption layer 18 and the second electrode 20, in order to adjust a band gap with respect to the light absorption layer 18. Further, an antireflection layer 26 is provided on the second electrode 20 in order to prevent light that enters the light absorption layer 18 from reflecting and leaking to the outside. The buffer layer 22, the high resistance layer 24, and the antireflection layer 26 are composed of, for example, CdS, ZnO, and MgF2 respectively. Further, ZnO or InS may be selected as the material for the buffer layer 22 in some cases. Either one of the buffer layer 22 or the high resistance layer 24 can be formed as a single layer.
A portion of the first electrode 16 is exposed from the stack 14, and a first lead section 28 is provided at the exposed portion. On the other hand, a portion of the second electrode 20 also is exposed from the antireflection layer 26, wherein a second lead section 30 is provided at the exposed portion.
When light, such as sunlight, is radiated onto the chalcopyrite type solar cell 10, which is constructed as described above, pairs of electrons and positive holes are generated within the light absorption layer 18. The electrons gather at the interface of the second electrode 20 (N-type side), whereas the positive holes gather at the interface of the light absorption layer 18 (P-type side), in relation to the joined interface between the CIGS light absorption layer 18 forming a P-type semiconductor, and the second electrode 20 forming an N-type semiconductor. When this phenomenon occurs, an electromotive force is generated between the light absorption layer 18 and the second electrode 20. Electrical energy brought about by the electromotive force is extracted externally as a current from the first lead section 28 and the second lead section 30, which are connected to the first electrode 16 and the second electrode 20 respectively.
Usually, the chalcopyrite type solar cell 10 shown in FIG. 5 is manufactured in the following manner. First, the first electrode 16 composed of Mo is formed as a film on a soda-lime glass substrate 12, for example, by means of sputtering film formation.
Subsequently, the first electrode 16 is divided by radiating a laser beam thereon, to perform a so-called “scribing” operation.
Cutting scraps, which occur during the dividing operation, are removed by washing with water. Thereafter, Cu, In, and Ga are caused to adhere onto the first electrode 16 by means of sputtering film formation, in order to provide a precursor. The precursor is placed in a heat treatment furnace, together with the substrate and the first electrode 16, to perform annealing in an H2Se gas atmosphere. The precursor is converted into selenide during the annealing process, thereby forming a light absorption layer 18 composed of CIGS.
Subsequently, the N-type buffer layer 22 composed of, for example, CdS, ZnO, or InS is provided on the light absorption layer 18. The buffer layer 22 is formed, for example, by means of sputtering film formation or chemical bath deposition (CBD).
Further, the high resistance layer 24 composed of, for example, ZnO is formed, for example, by means of sputtering film formation. Then, the high resistance layer 24, the buffer layer 22, and the light absorption layer 18 are subjected to scribing using a laser beam or a metal probe. As a result of scribing, the high resistance layer 24, the buffer layer 22, and the light absorption layer 18 are divided.
Subsequently, the second electrode 20 composed of ZnO/Al is provided by means of sputtering film formation. Then, the second electrode 20, the high resistance layer 24, the buffer layer 22, and the light absorption layer 18 are subjected to scribing using a laser beam or a metal probe.
Finally, the first lead section 28 and the second lead section 30 are provided at exposed portions of the first electrode 16 and the second electrode 20, respectively. Consequently, as a result of the aforementioned process, the chalcopyrite type solar cell 10 is obtained.
The chalcopyrite type solar cell 10, obtained as described above, forms one cell unit. Usually, a large-sized system having a panel-shaped form, in which a plurality of such cell units are electrically connected to one another, is used in practice.
As described above, glass is selected as the material for the substrate in most cases, since glass is easily available and inexpensive. In addition, since the glass surface itself is smooth, the surface of the film that is stacked on the substrate can also be made relatively smooth. Further, sodium contained in the glass diffuses toward the light absorption layer. As a result, energy conversion efficiency is increased.
However, when a glass substrate is used, and the precursor is subjected to selenide formation, high temperatures cannot be used. Therefore, it is difficult to advance selenide formation, so as to produce a composition in which the energy efficiency thereof is extremely large. Further, other inconveniences arise because the substrate is thick. Specifically, the feeding apparatus, which is used to feed the glass substrate when the chalcopyrite type solar cell is produced, must be large in size. Further, the produced chalcopyrite type solar cell has a large mass. Additionally, since the glass substrate is essentially non-flexible, it is difficult to adapt the aforementioned process to a “roll-to-roll process” mass production method.
As a countermeasure to overcome the above problems, materials for the substrate other than glass have been considered. For example, in Patent Document 1, a chalcopyrite type solar cell, in which a polymer film is used as a substrate, has been proposed. Additionally, in Patent Document 2, stainless steel has been proposed as a material for the substrate of a chalcopyrite type fuel cell. Patent Document 3 lists such materials as glass, alumina, mica, polyimide, molybdenum, tungsten, nickel, graphite, and stainless steel. Patent Document 2 also suggests providing a protective layer, composed of SiO2 or FeF2, in order to prevent the stainless steel substrate from being attacked by selenium during selenide formation.
Patent Document 1: Japanese Laid-Open Patent Publication No. 5-259494;
Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-339081;
Patent Document 3: Japanese Laid-Open Patent Publication No. 2000-58893.